1. Field of the Invention
The present invention generally relates to electrophotography apparatuses and exposure apparatuses, and more particularly to an electrophotography apparatus and an exposure apparatus which forms a latent image using an optical system of a LED head.
2. Description of the Related Art
Conventionally, there is an electrophotography apparatus which is provided with a LED head having a plurality of fine LED elements arranged in a main scanning direction. An image is formed on a recording medium such as paper by exposing a latent image on a drum using the LED head, developing the latent image, and transferring the developed image on the recording medium. The LED head of this electrophotography apparatus is designed so that a moving quantity amounting to 1 line in a sub scanning direction is equal to a width of the LED head in the sub scanning direction, and so that a scan in the sub scanning direction can be made at a finer pitch than in the main scanning direction. The sub scanning direction corresponds to a moving direction of the recording medium. The LED head prints the image on the recording according to the tone production method by density of each LED element, by making the halftone representation in the form of an area ratio representation. More particularly, the size of 1 pixel is divided into N elements in the sub scanning direction, and the gradation representation is made by the area ratio of the elements by successively scanning at the moving quantity corresponding to each of the N elements when making the exposure.
When a current is supplied to the LED head of the electrophotography apparatus to emit light, heat is generated in the LED head and the exposure width of the LED head changes. More particularly, the entire length of the LED head in the main scanning direction changes due to the generated heat. For this reason, in order to improve the resolving power and to improve the degree of the gradation representation, it is necessary to reduce the width of each LED element of the LED head in the sub scanning direction and to reduce the length of each LED element of the LED head in the main scanning direction. However, when the width and length of each LED element of the LED head are reduced, the light emitting area becomes small and the quantity of light decreases. In order to increase the quantity of light and to secure approximately the same exposure, it is necessary to supply more current to each LED element of the LED head. But when more current is supplied to each LED element of the LED head, the amount of heat generated from each LED element increases, thereby causing an error among the individual LED heads. The error in the pixel positions of the image exceeds a tolerable value when such an error is introduced among the individual LED heads, and an image having a high quality cannot be printed by use of such LED heads. As a result, it is not possible to make each LED element of the LED head small to the extreme.
Accordingly, in the electrophotography apparatus which prints a full color image using LED heads for the exposure of each of the colors, it is desirable to realize an optimum design wherein the amount of heat generated by the LED elements of the LED heads is small and the error among the printing positions of the images of each of the colors is eliminated, in a state where the resolving power of the images of each of the colors and the degree of the gradation representation are both improved. A contour of an object in the image should match among the images of each of the colors, but if the error exists among the printing positions of the images of each of the colors, the original color of the final image cannot be reproduced at the portion where the contours of the images of each of the colors do not match, thereby causing a color error or an unwanted color overlap. In this specification, such an error will be referred to as a registration error.
The following limitations exist in the case of the electrophotography apparatus which uses the LED heads and employs the tandem system.
(1) An error is introduced among the exposure widths of the LED heads, and thus, an error is generated in the printing positions of the LED heads in the main scanning direction. Hence, in order to suppress this error, It is necessary to reduce the amount of heat generated by the LED elements of the LED heads so as to become less than or equal to a tolerable value. For example, the amount of heat generated by the LED elements of the LED heads must be reduced so that the error among the entire lengths of the LED heads is less than or equal to a distance amounting to 1/2 pixel. On the other hand, the amount of heat generated by the LED elements of the LED heads becomes larger as the width of each LED element of the LED heads becomes smaller.
(2) Even if the width of the LED element of the LED head is reduced, the exposure beam diameter cannot be reduced beyond a certain limit if the same lens is used, where the same lens is gradient index (GRIN) lens such as a SELFOC lens.
(3) The width of the LED head in the sub scanning direction must be set smaller than the length of the LED head in the main scanning direction, and the moving quantity of the LED head in the sub scanning direction must be set small, so as to make the gradation representation by the area ratio representation.
On the other hand, the LED head is mounted on a frame, and radiator fins are secured on the frame by screws, so as to radiate the heat of the frame. But depending on the manner in which the radiator fins are secured on the frame by the screws or, when a central portion of the frame is pushed and bent for some reason such as a force applied on the central portion of the frame by a person mounting the LED head in the electrophotography apparatus, the relative positions of the radiator fins and the frame changes due to forced slipping of the radiator fins relative to the frame. When the relative positions of the radiator fins and the frame change, the relative positions will not return to the original positions because the radiator fins are secured on the frame by the screws. As a result, the frame on which the LED head is mounted remains in a state where the frame is bent by approximately several tens of .mu.m to one hundred and several tens of .mu.m. When the frame remains in such a bent state, there is a problem in that a defocusing (or out-of focus) state is generated.